Steel powder



Patented Jan. 30, 1945 UNITED STATES PATE T OFFICE No Drawing. Original application October 2,

1941, Serial No. 413,367. Divided and this application May 29, 1942, Serial No. 445,083

9 Claims. (01. 75-28) This invention relates to the production of steel powder, more particularly to an improved metal powder, produced from cheap source materials and available for powder metallurgy and other purposes. This application is a division of application Serial Number 413,367 filed October 2, 1941.

In the production of stainless steel, and particularly in the alloy steel casting industry, there is a considerable amount of .borings, turnings, and grinding residues available which contain not an inconsiderable percentage of stainless steel. These residues are produced, for the most part, by grinding stainless steel -or other high alloy castings with an abrasive wheel. The product or grinding residues produced is in quite finely divided form, but due to its contamination with the abrasive and bonding material of the wheel, it ha heretofore attained no commercial market.

In the field of powder metallurgy, stainless steel powder in various meshes has a consider sintered to produce an ultimate article of very satisfactory hysical characteristics.

As explained in that prior application, the stainless steel powder entering'these compacts steel scrap by suitable heat treatment followed by intergranular corrosion, ball milling or other disintegration treatment and classification,

It has now been found that a particularly useful and economical source material for stainless steel powder or other high alloy iron powders are the swing grinding residues produced by the final surfacing of alloy castings and similar alloy steel objects.

In the past this raw material, due .to the relatively high percentage of contaminated material, has had no commercial outlet. It has been found that this material may be treated by simple methods to produce a stainless steel or other alloy powder in desirably finely divided form and free from. associated contaminants. The product thus produced is particularly useful in certain fields, such as in the spraying of metal powder by the Schoop gun, in which very fine material of the order of 325 mesh is preferred.

The raw material which is subjected to treatment according to the present invention consists of grindings, borings, turnings, which contain the usual contaminants. This material contains quite a large percentage of such productsas silics, silicon carbide, organics (from the bonding agent of the grinding wheels), and the like. In most circumstances this material also contains quite a high percentage of ordinary iron, and sometimes brass.

In carrying out the invention this type of raw material is first heat treated in a special thermal 1 lowered within the carbide precipitation range to accelerate carbide precipitation. Due to this heat treatment carbides are precipitated in the grain boundaries and-slip planes; coincidentally, I

a 'considerable amount-of contaminants such as sulfur and the like are burnt off.

The product produced by theheat treatment is then quenched in a suitable contact corroding medium such as in a spent pickling liquor, from steel pickling, to. which liquor has been added of the order of 5% of sodium fluoride and sometimes up to 5% sulphuric acid. In thus quenching the hot material a considerable amount of may economically be produced from stainless the adherent abrasive is detached from the metal particles by exfoliation. Due to the contact with the pickling. liquor additional amounts of the abrasives are detached by reason of the under-. cutting effect of the pickle. Since the heat treated material is quenched in the pickle liquor the temperature of the latter is quickly elevated and the pickle is extremely active.

After thequenched product has been retained in the pickle liquor for a predetermined period of time, usually from about 30 minutes or more, the liquor is separated from the solid residue. This may be done in any suitable continuous filtering apparatus or by decanting off the supernatant pickle liquor.

The residue from the pickling stage may be washed to remove adherent pickle and is then screened. It'has been found that by a simple screening method a very large proportion of the oxides'and abrasives present in the original resi- I mesh screen and discarding the --100 or 200 mesh material. It has been found, similarly, that much of the low iron alloy content in the original residue will be removed in this screening operation, due to the fact that the low alloy iron is corroded to fines during the pickling, whereas the substantially corrosive-resistant stainless steel particles are unattacked.

The coarse material produced from the above screening operation comprises essentially stainless steel or other high alloy iron powder and is then disintegrated. This may effectively be done by crushing the material in a suitable type of hammer mill and preferably in a micro-pulverizer. This type of machine, as is known, can be set for a more or less definite degree of disintegration, i. e., for -100, -200, and -300 mesh material. The micro-pulverizer is peculiarly effective for this operation, for the material can be disintegrated at the rate of about 25 to 400 pounds per hour. In this type of apparatus, as those skilled in the art recognize, a series of stellite hammers move through a small distance, of the order of of an inch, at extremely high speed. It will be understood that in this disintegrating operation the material to be treated may be fed to the pulverizer either wet or dry or in admixture with any suitable lubricant. During this disintegrating.operation, due to the cold work applied to the metal, the alloys treated (18-8; 18-8-Mo; l8-8-Ti; etc.) are rendered magnetic.

The crushed material produced in the pulverizer step may then be treated in a suitable corrodent, so as to selectively attack the grain boundaries (high in carbides) and for removing imbedded abrasive scale, low alloy iron and the like. The corrodent for this step of the process may comprise a suitable aqueous solution of acid ferric sulphate and sodium fluoride. The period of retention in the corroding solution will vary depending upon the particular product treated, the temperature of the corroding solution, and the like.

The crushed and corroded material is now removed from the corrosion vat and is washed to free it of adherent corrodent. The product is then concentrated, preferably on a Welfiey table, to separate the metal particles from the non-metallic particles, i. e., abrasive material, oxides, carbides and the very fine unalloyed iron. These latter products are discarded in the tailings and the concentrate is recovered, which contains of the order of 90 to 98% of stainless steel powder.

If desired, the tailings, middlings and concentrate produced. in the above concentration step may be further concentrated either by flotation, electrostatic or magnetic separation.

The final concentrate, which comprises essentially an 18-8 steel powder, is then contacted or washed with a hot nitric acid solution between about and 20% concentration. This treatment serves a number of useful functions, namely, to brighten or shine such powder. The material thus produced, as noted previously, is particularly useful for spraying in metal guns of the type mentioned because of its desirably fine mesh, and for such purpose the product can be used directly.

The product may also be used for powder metallurgy purposes. However, for this use it is necessary to subject the material to an additional treatment, preferably to a dry hydrogen anneal in a temperature range between about 900 C. and 1200" C.

As will be appreciated by those skilled in the art, the heretofore waste material, namely, the grinding residues, may be processed to produce valuable alloy steel powder by modifications of the above method. Useful discrete steel powder may be produced from the described source material by first crushing the raw material in a micro-pulverizer, or similar machine, to such a degree that it passes through a 200 mesh screen. Since these metal particles are already quite severally work hardened (due to the grinding and boring operations which produce them) they are quite brittle and are readily disintegrated. During this grinding operation the contaminants in the raw material, 1. e., grinding oil, sulphur, rubber, Bakelite and the like, serve beneficially in the role of lubricants. During this grinding operation the attached or adhered abrasive particles are largely removed from the mesh by impact.

The ground product described above is then fed to a screen which classifies the material into two fractions, one of a screen analysis between 200 and 325 mesh and the other a -325 mesh. These two fractions are then each passed through a magnetic separator, or if desired each fraction may be treated by gravity separation methods such as flotation, Welfley table separation, and the like.

The magnetic concentrate, or the concentrate produced by table or flotation, is treated with a nitric acid solution of suitable concentration. This treatment serves to degrease the product by undercutting and to passivate the stainless steel. If the product is to be used for a remelting addition agent, in lieu of passivation, it may be treated with a typical pickling solution which serves to dissolve the sulphur and leviate the extreme fines of abrasives, oxides, carbides, Bakelite, and the like. This scum may then be removed from the top of the pickle bath.

The material thus produced is washed and dried and may be briquetted in any suitable manner, so as to be employed in a high alloy steel melt.

If the product is to be used as a pigment, for example, in paints, it is preferably ball milled either in a wet ball milling operation in contact with a liquid medium such as Varsol or it may be dry milled and polished with stearic acid.

It is to be observed that ii the product from the magnetic separator or other concentrator is too high in iron the nitric acid treatment or the pickling is continued so as to dissolve ordinary iron and steel.

If the material produced is to be used for spraying, it may be mixed with a flux or with a lower melting such for example as copper. Excellent results have been attained by mixing an 18-8 powder of the type described above with from about to 2% or more of copper powder. In these circumstances it is found that the sprayed metal coating quickly builds up on the sprayed base and is characterized by an extremelyhigh adhesion .and tenacity. Another particularly beneficial addition agent for this purpose is the eutectic powder described in my copending application. Serial No. 414,037. filed October 12, 1941. A particularly effective method of homogeneously incorporating the flux or the low melting metal powder in the stainless steel is to run the mixture through the micro-pulverizer. Another method is to separately grind the flux or low melting point metal powder so that its particle size will be equal to and preferably less than that of the stainless steel powder, and then mixing the two. For 18-8 spraying powders, in addition to copper and the iron eutectic powder mentioned the low melting calcium-manganese-silicon alloys, silver-copper-phosphorus eutectic alloy, ferromanganese and the like, may be utilized. Any metallic product compatible with stainless steel powder and which serves to accelerate or facilitate the bonding of the sprayed powder to the base material may be used.

After spraying the improved powder on the base material, for example on a low carbon steel base, the sprayed coating may be wire brushed to secure a desired matte finish or the material can be abraded fiat and then brushed, and if desired finished by polishing. Similarly, the sprayed material may be electrolytically polished.

In some circumstances where an extremely tenacious bond between the sprayed surface and the base plate is desired, it is advantageous to anneal the article under bright annealing conditions.

Spraying powder of the type above described is useful in a wide variety of fields, as, for example in the application of corrosion-resistant, relatively fine-sectioned surfaces to more corrodible base metal, and to the building up of steel surfaces, by the spraying technique, to a desired specification. In the event that highly wearresistant coatings are desired, the applied sprayed coating may be suitably treated, as by flame hardening, or high frequency hardening, to improve the wearing qualities of the applied coat.

I claim:

1. A method of producing stainless steel powder from stainless steel swing-grinding residues which comprises heat treating the residues in the carbide precipitation range for a period of time sufficient to cause intergranular carbide precipitation, quenching the material in a pickle liquor to largely remove the abrasive material, separating the 100 to -200 mesh material from the product and pulverizing the coarse material.

2. A method of producing stainless steel powder from stainless steel swing-grinding residues which comprises heat treating the residues in the carbide precipitation range for a period of time sufficient to cause intergranular carbide precipitation, quenching the material in a pickle liquor to largely remove the abrasive material, separating the -100 to -200 mesh material from the product, pulverizing the coarse material, and pickling the ground material to remove residual abrasive, carbides and iron.

3. A method of producing stainless steel powder from stainless steel grinding residues which comprises heat treating the residues under conditions regulated to drive ofi volatile material and to precipitate carbides in the grain boundaries of the stainless steel matrix, and separating the stainless steel metallic particles from abrasive and carbide material.

4. A method of producing stainless steel powder from stainless steel grinding residues which comprises heat treatin the residues under conditions regulated todrive ofl material which is volatile in the temperature range from 500 C. to

900 C. maintaining the elevated temperature for a period of time sufficient ,to cause intergranular carbide precipitation, treating the resulting product to separate abrasive material and carbides from the associated stainless steel particles, disintegrating the separated stainless steel particles, and treating such particles with a passivating agent.

5. A method of producing stainless steel powder from stainless steel grinding residues which comprises heat treating the residues in the carbide precipitation range for a period of time sufiicient to cause intergranular carbide precipitation and quenching the heat treated product in a contact 1 corrodent comprising a solution of ferrous sulphate containing sodium fluoride.

6. A method of producing stainless steel powder from contaminated stainless steel-containing residues which comprises heat treating the residues in the carbide precipitation range for a period of time sufficient to cause intergranular carbide precipitation; quenching the treated material in a contact corrodent; separating the solid material from the corrodent; screening the solid material to produce a fraction of approximately +200 mesh largely containing metallic material and a fraction of approximately 200 mesh consisting largely of non-metallic material; disintegrating the metallic material .in a suitable mill and subjecting the disintegrated material to the action of an intergranular corroding solution.

7. A method of producing stainless steel powder from contaminated stainless steel-containing residues which comprises heat treating the residues in the carbide precipitation range for a, period of time sufiicient to cause intergranular carbide precipitation; quenching the treated material in a contact corrodent; separating the solid material from the corroding solution; screening the solid material to produce a fraction consisting largely of metallic material and a fraction consisting largely of non-metallic material, disintegrating the metallic fraction; subjecting the disintegrated product to the action of a chemical solution which intergranularly corrodes the product and concentrating the corroded product on a Welfiey table to thereby remove residual non-metallics from the stainless steel powder.

8. A process in accordance with claim 7 in which the steel powder recovered is passivated in a nitric acid solution.

9. A method of producing stainless steel powder from contaminated stainless steel scrap which comprises heating the scrap material in the car- Y bide precipitation range for a period of time sufiicient to cause intergranular carbide precipitation; quenching the treated material in a solution of a contact corrodent to thereby remove a portion of the non-metallic contaminants; separating a clarified metal powder from the solution; screening the powder to produce a fraction enriched in metallic material, disintegrating such enriched fraction in a suitable mill; subjecting the disintegrated material to the action of a solution which intergranularly corrodes the stainless steel, and subjecting the corroded material to wet concentration to remove residual non-metal- 11c material. 1

JOHN WULFF. 

